This invention relates to a method of supplying a fuel into a gas turbine combustor, and more particularly, to a method of supplying a fuel into a two-stage combustion system gas turbine combustor of a low nitrogen oxide NO.sub.x to obtain an excellent combustion performance.
Since a combustion pressure and an inlet air temperature are high in a gas turbine combustor, the formation of NO.sub.x is generally greater than in boilers and heating furnaces, and hence reduction of NO.sub.x is an important problem to be solved as soon as possible.
NO.sub.x formed during the combustion process is affected by the combustion gas temperature, the oxygen partial pressure and the residence time of the gas, but among them, the influence of the combustion gas temperature is the greatest. It is therefore most effective to effect combustion at low temperature in order to reduce NO.sub.x. A so-called "lean combustion method" has been developed for reducing NO.sub.x in the gas turbine combustor, which method supplies air in a quantity exceeding the stoichiometrical quantity into the combustor and effects low temperature combustion. However, since the gas turbine combustor has an extremely wide operation range from the start to the rated load, the degree of lean combustion has further increased at the time of partial load if the reduction of NO.sub.x is accomplished by effecting sufficiently lean combustion at the time of the rated load. This results in the problems such as inferior combustion, increase of unburnt components, inferior ignition at the time of ignition, and so forth. For this reason, the degree of lean combustion is greatly restricted. In the case of high temperature high pressure combustion such as in the gas turbine, because the combustion speed is very fast, if so-called "diffusion combustion", which separately supplies the fuel and the air into the combustor for combustion, is employed, a major amount of combustion take place at a stoichiometric mixture or at a fuel rich mixture even though over all fuel to air supply ratio is not stoichiometric so that hot spot develops and NO.sub.x can not drastically be reduced.
Various combustion systems and constructions of the combustors have been examined so as to solve these technical problems. A so-called "two stage combustion system", which supplies separately the fuel to the upstream side and downstream side of the combustor has been examined in order to solve the former problem. Since the hot spot can be eliminated by eliminating the locally existing fuel rich zone, pre-mixing combustion has been examined in order to solve the latter problem. This means that pre-mixing combustion is an essential condition in order to drastically reduce the NO.sub.x in the gas turbine combustor, and, in this case, a two or multi-stage combustion is necessary so as to obtain excellent combustion performance throughout the entire operation range of the gas turbine.
In, for example, Japanese Patent Laid Open Application No. 112933/1980, a conventional gas turbine low NO.sub.x combustors is proposed, wherein the section of the combustor is narrowed so as to define first and second stage combustion zones, and the fuel is individually supplied to the first and second stage combustion zones. Furthermore, the feed of the fuel to the first stage is once cut off at its intermediate portion, and the fuel is again charged in order to provide the first stage combustion zone with the function of pre-mixing combustion.
Another prior art method effects stable diffusion combustion at the first stage and forms a lean fuel-air premixture of a pipe type at the second stage.
These prior art methods can drastically reduce the level of NO.sub.x at the time of high load, and can improve ignition and low load performance to some extent; however, the crucial problem of these prior art methods is that combustibility is deteriorated when the fuel is supplied at the second stage. More particularly, at the beginning of the second stage fuel supply period, the discharge of unburnt or uncombusted components increases extremely, the combustion flame of the second stage is so unstable so as to generate an oscillation combustion with blow-out of the flame at times, and the flame of the second stage interferring with the flame of the first stage, thereby oscillating the combustor as a whole. These problems occur because the fuel-air mixture ratio of the second stage combustion is so low at the period of the beginning of the second stage fuel supply, that a sufficiently stable combustion can not be maintained, combustion of the second stage does not smoothly proceed and the discharge of the unburnt or uncombusted components as well as oscillation combustion develop.
These problems can be solved, in principle, by any of the following methods.
(1) The design value of the fuel-air ratio of the second stage is made large.
(2) A difference is generated in the fuel concentration for the second stage combustion.
(3) The flowing direction of the combustible fuel-air mixture is set so as to come into contact and mix with the high temperature gas of the first stage combustion flame.
However, these methods remarkably increase the NO.sub.x from the second stage combustion flame and adversely affect the advantage of the two-stage combustion. In other words, it is a crucial problem in such two-stage low NO.sub.x combustors solve the problem of the deterioration of combustion performance when the fuel is supplied at the second stage.
The aim underlying the present invention essentially resides in providing a method of supplying a fuel to a gas turbine combustor which can reduce the NO.sub.x level without spoiling any other combustion performance.
In accordance with the present invention, a method is proposed which includes supplying a fuel to a gas turbine combustor operated by burning the fuel of the first stage and the air within a range from the start of the gas turbine until its low output range, and being operated by burning the fuels of both first and second stage within high output range of the gas turbine including its maximum output, with a predetermined quantity of fuel, in a step-wise means, being supplied to the second stage while at the same time, the same quantity of the fuel to be supplied to the first stage is decreased in a stepwise manner.
The fuel supply quantity to the second stage is greater than the quantity necessary for the instantaneous ignition of the second stage fuel, and a weight to air quantity ratio of the second stage is preferably greater than 0.01. This arrangement can accomplish the complete combustion when the fuel supplied at second stage as well as improving the stability of the second stage and can also improve the stability of the second stage combustion.
The adjustment of output in the high output operation range of the gas turbine is preferably accomplished by increasing or decreasing the quantity of the first stage fuel. This arrangement can restrict the occurrence of the unburnt or uncombusted components within the high output operation range.